1. Field of the Invention
The present invention relates to methods for producing semiconductor devices, and more specifically relates to methods for producing a metal-insulator-semiconductor field effect transistor (hereinafter, "MISFET") utilized generally in electronic instruments such as computers.
2. Prior Art
There are known MISFETs of the type composed of a gate electrode formed on a semiconductor region above a gate insulating film. In making this type of MISFET, conventionally, a field oxide film is formed on a surface of the semiconductor substrate by selective oxidation, and thereafter a gate oxide film and a polysilicon film are deposited sequentially. Then, the polysilicon film is patterned into a gate electrode, and a resist is coated over the field oxide film. Thereafter, an impurity such as P type boron is doped into a surface of the N type silicon substrate through the gate oxide film by ion implantation, using the patterned gate electrode as a mask so as to form a P+ type source region and a P+ type drain region.
However, the ion implantation method is conventionally used for forming the source and drain regions, thereby causing the following drawbacks:
1. The impurity is vertically injected in a Gausian distribution, hence a shallow impurity-doped region cannot be formed controllably. PA1 2. Accelerated ions of high energy bombard the substrate surface through the gate oxide film, thereby destroying the gate oxide film when its thickness is relatively small. PA1 3. Crystal defects may be caused on the semiconductor substrate surface due to ion bombardment. In view of the above noted drawbacks, it has proven difficult to produce a semiconductor device having a fast signal transfer speed and an ultra-small size.